The present invention relates to semiconductor fabrication and, more specifically, to an introduction of nickel to stabilize a cobalt liner or cap in copper interconnects.
Because copper (Cu) is a better conductor than aluminum, it has commonly been used for years in chips because metal components can be smaller. But copper can cause problems if not properly contained within the chip, so cobalt (Co) has been used for encapsulating copper as a liner along with a capping layer. The liner and cap enclose the copper, which can improve reliability by up to ten times or more.
However, cobalt is mobile in current 7-nm (nanometer) and 10-nm interconnects, both as a cobalt liner and as a cobalt cap. The movement/diffusion of cobalt has been linked with degraded electromigration lifetimes, particularly when a cobalt cap is combined with a ruthenium (Ru) liner.
Additionally, as exemplarily shown in FIG. 1, the divot 102 at the cobalt/copper interface after CMP (chemical mechanical planarization/polishing) causes poor dielectric cap coverage and can also degrade reliability by allowing oxygen in.
The present inventors have explored these cobalt mobility and divot issues and developed an approach to ameliorate these problems.